Define EC2 Instance Type with Allowed Values

EC2 Instance Type Variable

Controlling which EC2 instance types are allowed in your infrastructure prevents accidental cost overruns and ensures consistency across environments.

Variable Definition

variable "instance_type" {
  type        = string
  description = "EC2 instance type for the compute resources"
  default     = "t3.micro"

  validation {
    condition     = contains(["t3.micro", "t3.small", "t3.medium", "t3.large", "m5.large", "m5.xlarge"], var.instance_type)
    error_message = "Instance type must be one of: t3.micro, t3.small, t3.medium, t3.large, m5.large, m5.xlarge."
  }
}

Environment-Specific Values

# dev.tfvars
instance_type = "t3.micro"

# staging.tfvars
instance_type = "t3.medium"

# prod.tfvars
instance_type = "m5.large"

Why Use contains() Instead of Regex?

• Explicit allowlist: You see exactly which values are permitted
• Self-documenting: The error message lists all valid options
• Easy to update: Adding a new instance type is a one-line change
• No regex complexity: contains() is simpler and less error-prone

Cost Control Pattern

For organizations with budget constraints, combine this with a local map to enforce per-environment limits:

locals {
  allowed_types = {
    dev     = ["t3.micro", "t3.small"]
    staging = ["t3.small", "t3.medium"]
    prod    = ["m5.large", "m5.xlarge", "m5.2xlarge"]
  }
}

This pattern prevents developers from accidentally launching expensive instances in development environments.